Compliant microelectronic assemblies

ABSTRACT

A microelectronic assembly includes a microelectronic element having a first surface including a central region and a peripheral region surrounding the central region, the microelectronic element including a plurality of contacts disposed in the central region. The microelectronic assembly also includes a compliant layer over the peripheral region of the first surface, the compliant layer having a bottom surface facing toward the first surface of the microelectronic element, a top surface facing upwardly away from the microelectronic element and one or more edge surfaces extending between the top and bottom surfaces. A plurality of flexible bond ribbons are disposed over the compliant layer so that the bond ribbons extend over the top surface and one or more of the edge surfaces and the bond ribbons electrically connect the contacts to conductive terminals overlying the top surface of the compliant layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 09/071,412, filed May 1, 1998, now U.S. Pat. No. 6,284,563which is a continuation-in-part of U.S. patent application Ser. No.08/739,303, filed Oct. 29, 1996, now U.S. Pat. No. 6,211,572 which, inturn, claims benefit of U.S. Provisional Application No. 60/007,128,filed Oct. 31, 1995, the disclosures of which are hereby incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to semiconductor chip packaging. Moreparticularly, the present invention relates to an improved compliantsemiconductor package structure and methods for making the same.

FIELD OF THE INVENTION

The present invention relates to semiconductor chip packaging. Moreparticularly, the present invention relates to an improved compliantsemiconductor package structure and methods for making the same.

BACKGROUND OF THE INVENTION

Complex microelectronic devices such as modern semiconductor chipsrequire numerous connections to other electronic components. Forexample, a complex microprocessor chip may require many hundreds ofconnections to external devices.

Semiconductor chips commonly have been connected to electrical traces onmounting substrates by one of three methods: wire bonding, tapeautomated bonding, and flip-chip bonding. In wire bonding, the chip ispositioned on a substrate with a bottom or back surface of the chipabutting the substrate and with the contact-bearing front or top surfaceof the chip facing upwardly, away from the substrate. Individual gold oraluminum wires are connected between the contacts on the chip and padson the substrate. In tape automated bonding a flexible dielectric tapewith a prefabricated array of leads thereon is positioned over the chipand substrate and the individual leads are bonded to the contacts on thechip and to pads on the substrate. In both wire bonding and conventionaltape automated bonding, the pads on the substrate are arranged outsideof the area covered by the chip, so that the wires or leads fan out fromthe chip to the surrounding pads. The area covered by the subassembly asa whole is considerably larger than the area covered by the chip. Thismakes the entire assembly substantially larger than it otherwise wouldbe. Because the speed with which a microelectronic assembly can operateis inversely related to its size, this presents a serious drawback.Moreover, the wire bonding and tape automated bonding approaches aregenerally most workable with chips having contacts disposed in rowsextending along the periphery of the chip. They generally do not lendthemselves to use with chips having contacts disposed in a so-calledarea array, i.e., a grid-like pattern covering all or a substantialportion of the chip front surface.

In the flip-chip mounting technique, the contact bearing surface of thechip faces towards the substrate. Each contact on the chip is joined bya solder bond to the corresponding pad on the substrate, as bypositioning solder balls on the substrate or chip, juxtaposing the chipwith the substrate in the front-face-down orientation and momentarilymelting or reflowing the solder. The flip-chip technique yields acompact assembly, which occupies an area of the substrate no larger thanthe area of the chip itself. However, flip-chip assemblies suffer fromsignificant problems with thermal stress. The solder bonds between thechip contacts and substrate are substantially rigid. Changes in the sizeof the chip and of the substrate due to thermal expansion andcontraction in service create substantial stresses in these rigid bonds,which in turn can lead to fatigue failure of the bonds. Moreover, it isdifficult to test the chip before attaching it to the substrate, andhence difficult to maintain the required outgoing quality level in thefinished assembly, particularly where the assembly includes numerouschips.

Numerous attempts have been made to solve the foregoing problem. Usefulsolutions are disclosed in commonly assigned U.S. Pat. Nos. 5,148,265and 5,148,266. Preferred embodiments of the structures disclosed inthese patents incorporate flexible, sheet-like structures referred to as“interposers” or “chip carriers”. The preferred chip carriers have aplurality of terminals disposed on a flexible, sheet-like top layer. Inuse, the interposer is disposed on the front or contact bearing surfaceof the chip with the terminals facing upwardly, away from the chip. Theterminals are then connected to the contacts of the chip. Mostpreferably, this connection is made by bonding prefabricated leads onthe interposer to the chip contacts, using a tool engaged with the lead.The completed assembly is then connected to a substrate, as by bondingthe terminals of the chip carrier to the substrate. Because the leadsand the dielectric layer of the chip carrier are flexible, the terminalson the chip carrier can move relative to the contacts on the chipwithout imposing significant stresses on the bonds between the leads andthe chip, or on the bonds between the terminals and the substrate. Thus,the assembly can compensate for thermal effects. Moreover, the assemblymost preferably includes a compliant layer disposed between theterminals on the chip carrier and the face of the chip itself as, forexample, an elastomeric layer incorporated in the chip carrier anddisposed between the dielectric layer of the chip carrier and the chip.Such a compliant structure permits displacement of the individualterminals independently towards the chip. This permits effectiveengagement between the subassembly and a test fixture. Thus, a testfixture incorporating numerous electrical contacts can be engaged withall of the terminals in the subassembly despite minor variations in theheight of the terminals. The subassembly can be tested before it isbonded to a substrate so as to provide a tested, known, good part to thesubstrate assembly operation. This in turn provides very substantialeconomic and quality advantages.

Commonly owned U.S. Pat. No. 5,455,390 describes a further improvement.Components according to preferred embodiments of the '390 patent use aflexible, dielectric top sheet having top and bottom surfaces. Aplurality of terminals are mounted on the top sheet. A support layer isdisposed underneath the top sheet, the support layer having a bottomsurface remote from the top sheet. A plurality of electricallyconductive, elongated leads are connected to the terminals on the topsheet and extend generally side by side downwardly from the terminalsthrough the support layer. Each lead has a lower end at the bottomsurface of the support layer. The lower ends of the leads haveconductive bonding materials as, for example, eutectic bonding metals.The support layer surrounds and supports the leads.

Components of this type can be connected to microelectronic elementssuch as semiconductor chips or wafers by juxtaposing the bottom surfaceof the support layer with the contact-bearing surface of the chip so asto bring the lower ends of the leads into engagement with the contactson the chip, and then subjecting the assembly to elevated temperatureand pressure conditions. All of the lower ends of the leads bond to thecontacts on the chip substantially simultaneously. The bonded leadsconnect the terminals of the top sheet with the contacts on the chip.The support layer desirably is either formed from a relativelylow-modulus, compliant material, or else is removed and replaced afterthe lead bonding step with such a compliant material. In the finishedassembly, the terminals desirably are movable with respect to the chipto permit testing and to compensate for thermal effects. However, thecomponents and methods of the '390 patent provide further advantages,including the ability to make all of the bonds to the chip or othercomponent in a single lamination-like process step. The components andmethods of the '390 application are especially advantageous when usedwith chips or other microelectronic elements having contacts disposed inan area array.

Despite the positive results of the aforementioned commonly ownedinventions, still further improvements would be desirable.

SUMMARY OF THE INVENTION

The present invention contemplates a method of creating a compliantsemiconductor chip package assembly and the semiconductor chip packageassembly created therefrom.

In a fabrication process according to one aspect of the invention, afirst dielectric protective layer is provided on a contact bearingsurface of a semiconductor chip. The semiconductor chip has a centralregion bounded by the chip contacts and a set of apertures. Theapertures in the dielectric protective layer are provided such that thechip contacts are exposed. This first dielectric protective layer mayactually be the silicon dioxide passivation layer of the semiconductorchip.

Second, a compliant layer, preferably consisting of silicone,flexibilized epoxy, a thermosetting polymer or polyimide is providedatop the first dielectric protective layer is provided within thecentral region. The compliant layer is formed such that it has asubstantially flat top surface and edges that gradually slope down tothe top surface of the first dielectric protective layer. The slopingedges of the compliant layer may be manufactured to have a firsttransition region near the top surface of the compliant layer and asecond transition region near the bottom surface of the compliant layersuch that both the first transition region and the second transitionregion have a radius of curvature.

Finally, bond ribbons are selectively formed atop both the firstdielectric protective layer and the compliant layer such that each bondribbon electrically connects each chip contact to a respective terminalposition on the compliant layer. The bond ribbons may be selectivelyformed using a variety of techniques, such as by electroplating or byelectroless plating followed by selective etching. The terminalpositions are the conductive elements that connect the finished assemblyto a separate substrate, e.g. a printed circuit board.

The method described above may further include the step of providing fora second dielectric protective layer atop the bond ribbons and thecompliant layer after the bond ribbon electroplating step is performed.This optional second dielectric protective layer is fabricated with aset of apertures that expose the underlying terminal positions on thecompliant layer.

Additionally, the method described above may further include theoptional step of providing for an encapsulant layer above the bondribbons. If this optional step is performed, it is performed after thestep of selectively electroplating the bond ribbons. Like the firstdielectric layer, the encapsulant layer is fabricated with a set ofapertures so that the terminal positions are exposed. The encapsulantlayer material consists preferably of either a curable liquid, such assilicone, a flexibilized epoxy or a gel. This optional step may also beperformed just prior to the optional step of providing for a seconddielectric protective layer.

In another aspect of the invention, a method of making a compliantmicroelectronic assembly includes providing a microelectronic element,such as a semiconductor chip, having a first surface and a plurality ofcontacts disposed on the first surface thereof and forming a compliantlayer over the first surface of the microelectronic element. Thecompliant layer typically has a bottom surface facing toward the firstsurface of the microelectronic element, a top surface facing upwardlyaway from the microelectronic element and one or more edge surfacesextending between the top and bottom surfaces. The edge surfaces of thecompliant layer are preferably sloping surfaces which extend in bothvertical and horizontal directions. At least some of the sloping edgesurfaces preferably have first transition regions near the top surfaceof the compliant layer and second transition regions near the bottomsurface of the compliant layer, the first and second transition regionshaving respective radii of curvature.

In certain embodiments, before the compliant layer is formed, a firstdielectric protective layer, such as a layer including a silicon dioxidepassivation layer, may be provided on the first surface of themicroelectronic element. The first dielectric protective layer may havea plurality of apertures therein so that the contacts are accessibletherethrough. The compliant layer described above can then be providedover the dielectric protective layer.

Bond ribbons may then be selectively formed over the compliant layer.The bond ribbons preferably extend over both the top surface of thecompliant layer and one or more edge surfaces of the compliant layer.The bond ribbons electrically connect the contacts to conductiveterminals overlying the top surface of the compliant layer. Before thebond ribbons are formed, a barrier metal layer may be deposited over thecontacts so as to prevent undesired chemical reactions between thecontacts and the bond ribbons. In one embodiment, the bond ribbons areformed by selectively electroplating the bond ribbons atop the firstdielectric protective layer and the compliant layer. After the bondribbons have been formed, a dielectric cover layer may be formed overthe compliant layer and the bond ribbons. The dielectric cover layer mayhave a plurality of apertures therein so that the terminals areaccessible therethrough. In other embodiments, an encapsulant layer maybe provided atop the exposed surfaces of the bond ribbons. Theencapsulant layer is generally a material selected from the groupconsisting of silicone, flexibilized epoxy, thermoplastic and gel. Next,a second dielectric protective layer or cover layer may be provided overthe encapsulant layer. The second dielectric protective layer alsopreferably has a plurality of apertures therein so that the terminalsare accessible therethrough.

The compliant layer may include one or more apertures therein so thatthe contacts are accessible through the apertures. The one or moreapertures may include one or more groups of apertures encompassing aplurality of the contacts. The edge surfaces of the compliant layer mayinclude one or more aperture edge surfaces bounding the apertures, withat least some of the bond ribbons being formed over the aperture edgesurfaces. The compliant layer may be formed by engaging themicroelectronic element with a mold so that one or more projections onthe mold contact the first surface of the microelectronic element. Aflowable composition may be introduced around the projections and theflowable composition set to provide a compliant layer. Themicroelectronic layer may then be separated from the mold. The one ormore apertures are typically formed in the space occupied by theprojections.

In certain embodiments, the contacts on the microelectronic element aredisposed in an area array, and the one or more apertures in thecompliant layer include a plurality of apertures disposed in an arraycorresponding to the array of contacts so that each contact isencompassed in a respective aperture. In other embodiments, the contactson the microelectronic element may be disposed in a first region of thefirst surface, with the compliant layer overlying a second region of thefirst surface, and one or more edge surfaces including one or moreborder edge surfaces extending along one or more borders between thefirst and second regions. In still other embodiments, the contacts onthe microelectronic element are disposed in a central region of thefirst surface and the compliant layer overlies a peripheral region ofthe first surface.

In another embodiment, a method of making a compliant microelectronicpackage includes providing a supporting element having anupwardly-facing top surface and juxtaposing a microelectronic elementincluding a first surface having a plurality of contacts thereon withthe supporting element so that the first surface of the microelectronicelement is disposed alongside the top surface of the supporting element.The first surface of the microelectronic element and the top surface ofthe supporting element may be substantially coplanar after thejuxtaposing step.

A compliant layer may then be provided over the top surface of thesupporting element, the compliant layer having a top surface remote fromthe top surface of the supporting element, a bottom surface and an edgesurface extending between the top surface and the bottom surface. Incertain embodiments, a portion of the compliant layer extends over thefirst surface of the microelectronic element, with one or more edgesurfaces of the compliant layer overlying the first surface of themicroelectronic element. Bond ribbons may then be selectively formedatop the compliant layer, the bond ribbons electrically interconnectingthe contacts of the microelectronic element with conductive terminalsoverlying the top surface of the compliant layer.

The supporting structure described above may have a central aperturetherein so that the microelectronic element may be placed in the centralaperture after being juxtaposed with the supporting element. After thejuxtaposing step, the first surface of the microelectronic element andthe top surface of the supporting structure are preferably substantiallycoplanar.

In certain embodiments, the compliant chip assembly may include a groundplane electrically interconnected with at least one of the bond ribbons.The ground plane may include a plurality of apertures therein so thatthe terminals are accessible through the apertures.

The methods described above can be applied simultaneously to amultiplicity of undiced semiconductor chips on a wafer to form acorresponding multiplicity of compliant semiconductor chip packages.After the bond ribbons have been formed on the packages, individualpackages may be severed or diced from the wafer to provide separate anddistinct chip packages. The methods may also be applied to amultiplicity of adjacent semiconductor chips arranged in an array toform a corresponding multiplicity of compliant semiconductor chippackages, whereby the packages are diced after the bond ribbons havebeen formed.

A further aspect of the present invention includes the structure of aunique compliant semiconductor chip package having fan-in type leads.The compliant semiconductor chip package is comprised of (1) asemiconductor chip having a plurality of peripheral bonding pads on aface surface thereof and a central region bound by the peripheralbonding pads; (2) a first dielectric protective layer having a firstsurface, a second surface and apertures, wherein the first surface ofthe first dielectric layer is joined to the face surface of thesemiconductor chip and the peripheral bonding pads are exposed throughthe apertures; (3) a compliant layer having a top surface and a bottomsurface, wherein the bottom surface of the compliant layer is joined tothe second surface of the first dielectric layer within the centralregion of the semiconductor chip package; and (4) a plurality ofelectrically conductive bond ribbons, each bond ribbon having a firstend that electrically couples to a respective peripheral bonding pad ofthe semiconductor chip and a second end that joins to the top surface ofthe compliant layer to form a package terminal.

The package terminals of the completed package are configured in anarray that has an area smaller than the area bound by the peripheralbonding pads on the face of the semiconductor chip. In other words, thepackage has fan-in leads that permits minimization of the overallpackage size.

For increased reliability, the compliant layer has sloped peripheraledges so that the overlying bond ribbons are curved rather than kinked.

The compliant semiconductor chip package may also have a compliant layercharacterized by an array of bumped protrusions. The bumped protrusionssupport the overlying conductive terminal position ends of the bondribbons and function as conductive balls that join to a substrate thusforming a ball grid array type interconnection. Alternate to the bumpedprotrusions, the compliant layer may have an array of concavities thatare useful for placement of solder balls into each concavity. Thisarrangement is also useful for a ball grid array type interconnect.

The foregoing and other objects and advantages of the present inventionwill be better understood from the following Detailed Description of aPreferred Embodiment, taken together with the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a semiconductor chip assembly atthe beginning of a fabrication process.

FIG. 1B is a cross-sectional view of the semiconductor chip assemblyafter a first step of the fabrication process, showing a deposited orlaminated dielectric passivation layer.

FIG. 1C is a cross-sectional view of the semiconductor chip assemblyafter a second step of the fabrication process, showing a deposited orlaminated compliant layer within the central region of the semiconductorchip contact-bearing surface.

FIG. 1D is a cross-sectional view of the semiconductor chip assemblyafter a third step of the fabrication process, showing a conductive seedlayer that has been sputtered over the assembly.

FIG. 1E is a cross-sectional view of the semiconductor chip assemblyafter a fourth step of the fabrication process, illustrating how after aphotolithographic step conductive bond ribbons can be formed over theassembly.

FIG. 1F is a cross-sectional view of the semiconductor chip assemblyafter a fifth step of the fabrication process, showing how the assemblyis coated with a second dielectric protective layer.

FIG. 2 is a perspective view of the semiconductor chip assembly afterthe bond ribbons have been formed over the compliant layer but beforethe second dielectric protective layer is coated.

FIG. 3 is a plan view of a wafer having a multiplicity of semiconductorchips, illustrating how said multiplicity of semiconductor chips can besimultaneously packaged using the semiconductor chip assembly processdepicted in FIGS. 1A-1F.

FIG. 4 is a cross-sectional view of an alternate embodiment of thepresent invention, illustrating the use of a low modulus encapsulantmaterial to provide further support and stress relief to the bondribbons.

FIG. 5A is a cross-sectional view of an alternate embodiment of thepresent invention, illustrating the formation of bumped protrusions inthe compliant layer that raise the overlying terminals such that theterminals form an array over the top surface of the compliant layer.

FIG. 5B is a perspective view of the embodiment shown in FIG. 5A.

FIG. 6A is a cross-sectional view of an alternate embodiment of thepresent invention, illustrating the formation of concave areas in thecompliant layer such that the overlying terminals have cup-likedepressions useful for accurate placement of solder balls.

FIG. 6B is a perspective view of the embodiment shown in FIG. 6A.

FIG. 7A is a cross-sectional view of a first step of a semiconductorchip assembly process according to another embodiment of the presentinvention.

FIG. 7B is a cross-sectional view of the assembly shown in FIG. 7A,showing a mold for forming a compliant layer on top of the assembly.

FIG. 7C is a cross-sectional view of the assembly shown in FIG. 7B afterconductive bond ribbons have been formed atop the compliant layer.

FIG. 7D shows the assembly of FIG. 7C after the top of the assembly hasbeen coated with an additional dielectric protective layer.

FIG. 8A is a perspective view of the semiconductor chip assembly shownin FIG. 7C, before the additional dielectric protective layer has beenprovided over the bond ribbons.

FIG. 8B is a close-up, fragmentary, cross-sectional view of the assemblyshown in FIG. 7D.

FIG. 9 is a cross-sectional view of another embodiment of the presentinvention which includes a semiconductor chip having a plurality ofcontacts in a central region thereof.

FIG. 10 is a perspective view of the assembly shown in FIG. 9.

FIG. 11 is a top view of a semiconductor chip having a plurality ofnon-uniform, staggered chip contacts in a peripheral region of asemiconductor chip, in accordance with another embodiment of the presentinvention.

FIG. 12 is a fragmentary top view of the chip shown in FIG. 11 after acompliant layer and bond ribbons have been formed atop the chip.

FIG. 13 is a cross-sectional view of a compliant chip assembly having asupporting element with a central opening and a semiconductor chipprovided in the central opening of the supporting element in accordancewith yet another embodiment of the present invention.

FIG. 14 is a cross-sectional view of a compliant chip assembly includinga flexible dielectric sheet in accordance with still another embodimentof the present invention.

FIG. 15 is a cross-sectional view of a compliant chip assembly includinga ground plane in accordance with a further embodiment of the presentinvention.

FIG. 16 is a fragmentary top view of the compliant chip assembly shownin FIG. 15.

FIG. 17 is a cross-sectional view of a compliant chip assembly includinga ground plane in accordance with still further embodiments of thepresent invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1A-F illustrate a side view of the process of creating thecompliant chip package of the present invention on the face surface of asingle die, on the face surfaces of multiple die arranged in a coplanararray or on the face surface of an undiced silicon wafer which may besubsequently diced into individual packaged chips or multi-chip modules.

FIG. 1A shows a single semiconductor chip 100 with a contact bearingface surface 120. The contacts 110 on the face surface 120 are typicallyaligned in a peripheral region 112 and further define a central region115 therein. In FIG. 1B, a dielectric passivation layer is deposited oradhered onto the face surface 120 of the chip 100. The passivation layermay simply be the SiO₂ passivation layer (not shown) commonly found onthe contact bearing surface of semiconductor chips, or a separatedielectric passivation layer 130 may be used, such as an epoxy resin, apolyimide resin, photo-imagable dielectric, etc. If the separatepassivation layer 130 is used, the passivation layer 130 may be spunonto and built up to a planar sheet-like form on the face surface 120 ora dielectric sheet may be laminated to the face surface 120 using any ofa number of electronic grade adhesives commonly known and used by thoseskilled in the art. The passivation layer 130 covers the face surface120 of the chip 100 while leaving the chip contacts 110 exposed so thata bond ribbon may be plated thereon in a later step, as described below.Typically, this will be done by depositing or adhering the passivationlayer 130 in a continuous sheet on the face surface 120 of the chip 100.A registering system, such as an automatic vision system, is used tolocate the contacts 110. If a photo-imagable dielectric is used, thepassivation layer 130 may be exposed and developed without exposing thearea above the contacts 110, that unexposed area may then be removed.Another removal process which can be used is to use a pulse of directedenergy, such as an excimer laser, to selectively remove the passivationlayer 130 above the contacts 110. Alternately, a continuous dielectricsheet already having set contact holes may be registered and laminatedto the chip 100.

In the next step, as illustrated in FIG. 1C, a compliant layer 140 isdeposited or laminated onto the exposed surface of the passivation layer130. The compliant layer 140 may be stenciled, screened or transfermolded onto the passivation layer 130 using a curable liquid which, whencured, adheres to the passivation layer 130. Alternately, the compliantlayer 140 may be adhered to the exposed surface of the passivation layer130 in the form of cured compliant pads using the aforementionedelectronic grade adhesives. The compliant layer 140 has a substantiallyflat top surface 147 which further typically has a gradual, slopingtransition 145 between the face surface 120 of the chip 100 and the topsurface 147. This transition 145 may follow a line of curvature from thepassivation layer 130 to a substantially flat top surface 147 or maysimply be canted at an angle such that the transition 145 is not toovertically oriented in relation to the passivation layer 130 and the topsurface 147. The compliant layer 140 itself may be formed from a widevariety of materials; however, preferably, a low modulus of elasticitymaterial is used as the compliant layer 140. Compliant interposerstypically are fabricated from polymeric and other materials such assilicones, flexibilized epoxy, polyimides and other thermosettingpolymers, fluoropolymers and thermoplastic polymers. Also, theinterposer may be a composite incorporating plural materials. Theinterposer may consist of, or incorporate, a foam or mesh layer. Theflexibility of the interposer depends on the thickness and configurationof the interposer, as well as on the properties of the materials usedtherein. Thus, a flexible interposer, capable of buckling or wrinklingto accommodate relative movement, can be fabricated from high elasticmodulus materials, normally considered as “rigid” provided that thesematerials are present in thin layers. Relatively soft materials andfoams can be used in greater thicknesses and still provide a highlyflexible interposer. Moreover, such soft materials and foams provide ahighly compliant interposer, i.e., an interposer which is readilycompressible in the directions perpendicular its surfaces and whichtherefore permits movement of the terminals in these directions.

A plating seed layer 150 is then deposited atop the aforementionedassembly, as shown in FIG. 1D, typically using a sputtering operation.Typical plating seed layer materials include palladium (for electrolessplating), titanium, tungsten, nickel, chromium; however, primarilycopper seed layers are used. FIG. 1E shows the next step in whichphotoresist 160 is applied to the exposed top surfaces of the assemblyand then exposed and developed such that bond ribbons 170 may be platedwithin defined areas to form conductive paths electrically connectingthe chip contacts 110 near a first end region of the ribbons 170 toterminals 175 comprising the second end region of the ribbons 170. Thisis perhaps more easily seen in the perspective view shown in FIG. 2. Asshown, the ribbons 170 are plated directly onto the contacts 110 andextend in a “fan-in” arrangement from the peripheral region 112 to thecentral region 115 of the face surface 120 of the chip 100 atop thecompliant layer 140. Possible bond ribbon materials include copper,gold, nickel, and alloys, combinations and composites thereof, amongothers. Since the bond ribbons 170 are plated directly onto the chipcontact/compliant layer themselves, there is no need to develop aprocess for bonding the ribbons 170 to the contacts, as is necessarywith most other approaches such as TAB, beam lead or wirebonding. Thisprovides a significant cost savings because specializedthermocompression or ultrasonic bonders and their bonding tools need notbe purchased or maintained. It is important, however, that the materialselected for the bond ribbon 170 be compatible with the chip contact 110material, which is typically aluminum. Otherwise, a phenomenon calledKirkendahl Voiding (voids created at the boundary of two metals havingdifferent interdiffusion coefficients) may cause voiding along theboundary of the two metals (ribbon/contact) leading to intermetallicdegradation and embrittlement of the bond ribbon 170 itself making thelead/bond susceptible to failure during thermal cycling. Alternately,one or more barrier metals may be plated atop the chip contacts 110prior to the bond ribbon plating step to thereby ensure thecompatibility of materials.

As shown in FIG. 1F, preferably, a dielectric layer 180 is deposited orlaminated over the top of the assembly so that only the terminals 175are exposed. The dielectric layer may be comprised of a screened,exposed and developed or laminated sheet photo resist material or may becomprised of paralyne, epoxy resin, polyimide resin, fluoropolymer, etc.which is deposited or laminated on to the assembly, as described abovein relation to the passivation layer 130. The terminals 175 may then beelectrically connected to a circuitized substrate, such as a printedwiring board.

Typically, a solder ball or a solid-core solder ball will be used tocreate this electrical connection. The dielectric layer 180 is thus usedas a solder mask to ensure that the solder does not electrically shortbetween adjacent bond ribbons 170. Oxide layers and other surfacecontaminates typically build up on the surface of many types of metal(copper, nickel, etc.). Although not shown in FIG. 1F, the terminals 175are typically flash plated with a thin layer of gold (approximately 0.25to 0.5 microns) to inhibit the formation of these oxide layers. The goldlayer is kept very thin so that it does not appreciably affect theaforementioned solder joint by dissolving into the solder to an amountwhich would embrittle the resulting solder joint between the terminaland a circuitized substrate.

The configuration of the above described chip package allows the packageto mechanically decouple the chip 100 from an attached circuitizedsubstrate (not shown). Typically, solder connections between the chipand the circuitized substrate are woefully inadequate to compensate forthe thermal mismatch problem during temperature cycling of the chip. Thecombination of the compliant layer 140 and the flexible bond ribbonsplated thereon allow the package to compensate for much of the TCEmismatch problem by giving limited movement of the terminals in the X, Yand Z directions with respect to the chip contacts 110 therebyminimizing the stress placed on the solder connections themselves,without imposing substantial forces on the bond between the ribbons 170and the chip contacts 110. Further, because the compliant layer 140 iscompressible, it also has the effect of compensating for any terminals175 which are not perfectly planar with respect to its adjacentterminals when the terminals 175 are abutted against and coupled to thecircuitized substrate. However, the top surface 147 of the compliantlayer 140 should be made as flat and planar as possible so that theterminals 175 all lie in or near the same plane in order to minimize theamount of pressure needed to be placed on the bottom surface 125 of thechip 100 to ensure that all of the terminals/solder balls areelectrically connected to a circuitized substrate.

As illustrated in FIG. 3, the chip package described above in relationto FIGS. 1 and 2 may also be provided in the form of a multiplicity ofpackages on a wafer incorporating a plurality of individual, undicedchips, all of the same design or of differing designs. As shown, anarray of individual passivation layers 230 may be deposited or laminatedonto the face surface 220 of the wafer 200 leaving the chip contacts 210of the various individual chips exposed, as described above. Thisarrangement is shown to better define the individual chips within thewafer. Preferably, however, a single passivation layer 230 is depositedor laminated onto the face surface 220 leaving the contacts 210 exposed.Individual compliant layers 240, as described above, are deposited orlaminated onto the central regions of each of the individual chipswithin the wafer 200. The steps found in FIG. 1A-F are then performed,as described above, to create a plurality of connected individuallypackaged chips on the face surface 220 of the wafer 200. Each packagedchip having bond ribbons 270 which are connected at one end to contacts210 and extending in to a central region of the respective chip in afan-in fashion atop a respective compliant layer 240 and ending with aterminal 275 on the top surface 247 of the compliant layer 240. Afterthe individual packages are completed, the individual chips may beseparated from the wafer 200 and from one another, as by cutting thewafer 200 using conventional wafer severing or “dicing” equipmentcommonly utilized to sever wafers into individual chips. This procedureyields a plurality of packaged chip subassemblies, each of which may besecured to an individual circuitized substrate. Alternately, the chipsmay be separated from the wafer 200 in multi-chip arrangements ofmultiples of the same or different operational chips. The wafer levelembodiment shown in FIG. 3 could be simulated using a panel ofindividual chips spaced apart from one another in a processing boat. Theface surfaces of the individual chips would be coplanar with respect toone another to simulate the face surface 220 of the wafer 200. The chipsabove described steps would be performed and the chips would beseparated if desired.

In the alternate embodiment shown in FIG. 4, a low modulus encapsulantmaterial 290 may be deposited around the exposed surfaces of the bondribbons 170′ leads prior to the step shown in FIG. 1F of depositing orlaminating the assembly with the dielectric layer 180′. The encapsulantmaterial 290 may have properties similar to those of rubber, gum or gel.Typical encapsulation materials include curable liquid or cured padscomprised of silicone, flexibilized epoxy, gels, thermoplastics, etc. Ifthe encapsulant 290 is applied as a curable liquid, a fixture may bemade such that the liquid flows around the bond ribbons 170′ but doesnot flow on top of the terminals 175′ to ensure that solder balls may besubsequently electrically connected to the terminals 175′, as describedabove. Alternately, a machine such as a Camalot 1818 manufactured byCamalot Systems, Inc. of Havermill, Mass. may be used to flow the liquidencapsulant into the desired areas. After the liquid is deposited, itmay be cured by any number of ways depending on the encapsulant material290 used, e.g. heat, infrared energy, etc. The encapsulant 290 giveseach of the bond ribbons 170′ more support and further spreads some ofthe stress away from the ribbons 170′ thus allowing a larger TCEmismatch between the chip and a circuitized substrate, as describedabove. After curing of the encapsulant 290, the dielectric layer 180′may be deposited or laminated thereto.

In another alternate embodiment, a conductive material such as berylliumcopper, or a super plastic or shape memory alloy (such as Nitinol), issputtered or otherwise deposited across the entire exposed surface ofthe chip/passivation layer/compliant layer (100/130/140) combination,shown in FIG. 1C. The conductive material may then be etched usingindustry standard photolithographic techniques resulting in amultiplicity of bond ribbons positioned and configured much like thebond ribbons 170 shown in FIG. 1E and FIG. 2. In this embodiment, asdescribed above, a barrier metal, such as a flash plated layer of gold,may first be plated to the chip contacts to ensure compatibility of theelectrical connection between the chip contact and the bond ribbon.Likewise, a flash plated layer of gold may be plated atop the exposedsurface of the terminal. Also, the entire exposed surface of the bondribbon could be plated with a thin layer of gold to increase the overallconductivity of such super plastic leads. A dielectric layer is nextdeposited or laminated as shown in FIG. 1F.

FIG. 5A shows a side view and FIG. 5B a perspective view of anotherembodiment, according to the present invention. In this embodiment, thecompliant layer 140′ has protrusions 300 on its top surface 140′. Theseprotrusions 300 may be integral with the compliant layer 140′ or may bedeposited or laminated onto the top surface 140′ subsequent to theformation of the compliant layer 140′. The protrusions 300 may be formedof compliant, elastomeric material, such as the material comprising thecompliant layer 140′, or may be comprised of a semi-rigid or rigidmaterial. The bond ribbon terminals 175′ are plated on top of theprotrusions 300 thereby providing raised surfaces which may be connectedto a circuitized substrate. This technique allows for connection to sucha substrate using less solder and without the need to accuratelyposition solid-core solder balls.

FIG. 6A shows a side view and FIG. 6B a perspective view of anotherembodiment, according to the present invention. In this embodiment,concave areas 310 are created in the compliant layer 140″. These concaveareas 310 may be create in the formation of the compliant layer 140″ ormay be created subsequent to the formation of the compliant layer 140″.The bond ribbon terminals 175″ are plated within the concave areas 310creating conductive “cup-like” areas on the top surface 147″ of thecompliant layer 140″. Solder or solid-core solder balls are then placedwithin these areas 310 and reflowed to attach the package to acircuitized substrate, as described earlier. This technique allows forthe accurate placement of solder or solid-core solder balls by allowingthem to be deposited and retained within the cup-like areas.

FIGS. 7A-7D illustrate a side view of a method of making a compliantmicroelectronic package including a semiconductor chip having an areaarray of contacts on a first surface thereof. The package is preferablyassembled by using the method steps described above.

FIG. 7A shows a single semiconductor chip 400 having a first surface 420including a plurality of contacts 410 provided in an area array over thefirst surface 400. A dielectric passivation layer 430 is deposited overthe first surface 420 of the chip 400 and preferably covers the firstsurface 420 of the chip 400 while leaving the chip contacts 410 exposedso that a bond ribbon (not shown) may be plated thereon, as will bedescribed in more detail below.

Next, as illustrated in FIG. 7B, the chip 400, including the passivationlayer 430, is placed in a mold 488 so that a compliant layer may beformed atop the passivation layer 430. The compliant layer 440 ispreferably molded onto the passivation layer 430 using a curable liquidwhich, when cured, adheres to the passivation layer 430. In onepreferred embodiment, the mold 488 has downwardly extending projections489 which are shaped to completely cover the chip contacts 410 when themold 488 is in a closed position. The mold 488 includes open spaces 493between the projections 489. In order to form the compliant layer 440,the chip 400 is placed in a frame 491 and the mold is closed on top ofthe chip 400 so that the projections 489 completely cover the contacts410. Next, a curable liquid 440 is introduced into the mold and fillsthe open spaces 493 between the projections 489. The curable liquid isthen cured while the mold remains in the closed position so as to formthe compliant layer 440 having a substantially flat top surface 447including a plurality of openings 495 aligned with the contacts 410. Theheight of projections 489 is exaggerated in FIG. 7 for clarity ofillustration. In practice, projections 489 typically are about 75-200microns high, and hence compliant layer 440 typically is about 74-200microns thick. In each opening 495 has a gradual sloping edge 497 ortransition between the first surface 420 of the chip 400 and the topsurface 447 of the compliant layer 440. This sloping edge 497 willpreferably follow a line of curvature from the passivation layer 430 tothe substantially flat top surface 447, or may simply be canted at anangle such that the sloping edge 497 is not too vertically oriented inrelation to the passivation layer 430 and the top surface 447 of thecompliant layer. For example, sloping edge 497 typically is disposed atan angle of about 20-70° to the plane of the chip front surface, andmore typically about 40-60°. Also, the sloping surface typically iscurved to define a radius at the juncture of sloping surface 497 and topsurface 447. A further radius or fillet can be provided at the junctionof the sloping surface and the front surface of the chip.

In the next step, illustrated in FIG. 7C, bond ribbons 470 areselectively formed within defined areas to create conductive pathselectrically connecting the chip contacts 410 near a first end of thebond ribbons 470 to conductive terminals 475 at a second end of the bondribbons. In certain embodiments, the bond ribbons 470 may be formedusing selective electroplating or other selective deposition techniques.In other embodiments, the selection forming step used to make bondribbons 470 may include one or more nonselective deposition techniquessuch as electroless plating or sputtering of a conductive layer over theassembly, with or without an additional non-selective electroplatingstep, followed by selectively etching of the conductive layer to provideelectrically isolated bond ribbons. FIG. 8A shows a perspective view ofthe bond ribbons after they have been selectively formed over thecompliant layer. In alternative embodiments, one or more barrier metallayers (not shown) may be plated atop the chip contacts 410 prior toforming the bond ribbons 470 so as to insure the compatibility ofmaterials.

Referring to FIG. 7D, a dielectric layer 480 is then deposited orlaminated over the top of the assembly so that only the conductiveterminals 475 are accessible at the top of the assembly. The terminals475 may then be electrically interconnected with an external circuitelement, such as a printed circuit board. Typically, a solder ball orsolid core solder ball will be used to create this electricalconnection. Thus, the dielectric layer 480 serves as a solder mask,thereby insuring that the solder does not electrically short betweenadjacent bond ribbons 470.

FIG. 8B shows a close-up, fragmentary, cross-sectional view of FIG. 7D.The assembly includes the compliant layer 440 having a plurality ofapertures 495 therein so the contacts 410 are accessible through theapertures 495. Each aperture 495 in the compliant layer 440 preferablyincludes at least one sloping edge side wall 497 which provides agradual sloping transition between the first surface 420 of the chip 400and the top surface 447 of the compliant layer 440. The transitionpreferably follows a line of curvature from the passivation layer 430 tothe top surface 447 or may simply be canted at an angle so that thetransition from the first surface 420 of the chip 400 to the top surface447 of the compliant layer 440 is not too vertically oriented inrelation to the passivation layer 430. The top surface 447 of thepassivation layer is preferably substantially flat, however, in certainembodiments the top surface 447 may be slightly rounded. As statedabove, the low points in the compliant layer may next be filled withcompliant material to encase the leads and/or cover sheets of material.

As illustrated in FIG. 9, a compliant chip package in accordance withanother preferred embodiment of the present invention includes a singlesemiconductor chip 500 having a first surface 520 with a first orcentral region 515 and a second or peripheral region 517 surrounding thecentral region 515. The chip 500 includes a plurality of contacts 510disposed in the central region 515 thereof. A passivation layer 530 ispreferably deposited over the first surface 520 of the chip 500. Thepassivation layer 530 includes apertures aligned with the contacts 510so that the chip contacts 510 are accessible through the passivationlayer 530. A compliant layer 540 is then formed over the passivationlayer, the compliant layer having openings 595 in alignment with thechip contacts 510 so that the contacts are accessible through thecompliant layer openings 595. The steps described above are thenperformed to create a plurality of bond ribbons 570 which are connectedat one end to the chip contacts 510 and at a second end to conductiveterminals 575 accessible at the substantially flat surface 547 of thecompliant layer 540. The final assembly provides a compliant chippackage having a plurality of contacts 510 in the central region 515thereof and bond ribbons 570 extending outwardly from the contacts 510to conductive terminals 575 overlying the peripheral region 517 of thechip 500. The centrally located low point in the compliant layer can befilled in with compliant material to encapsulate the leads.

FIG. 10 shows a perspective view of the package illustrated in FIG. 9.As shown in FIG. 10, the plurality of contacts 510 are located in thecentral region 515 of the chip 500. Compliant layer 540 defines twosloping edges 572 at the border of the first or central region of thechip surface and the second or peripheral region. Bond ribbons 570 havefirst ends electrically connected to the contacts 510 and second endsextending to conductive terminals 575 provided at the top surface 547 ofthe compliant layer 540. The specific embodiment shown in FIG. 10includes a compliant layer 540 having a first section on the left sideof the chip 500 and a second section on the right side of the chip 500,however, other preferred embodiments may include compliant layers havingmore than two distinct portions.

In still another embodiment, illustrated in FIG. 11, a compliant chippackage includes a semiconductor chip 600 having a first surface with acentral region 615 and a peripheral region 612 surrounding the centralregion 615. The peripheral region 612 includes a plurality of contacts610 which are arranged in a staggered or non-uniform configuration. Inother words, the peripheral region 612 includes contacts 610 which arepositioned at non-uniform distances from an edge 617 of the chip 600. Inother embodiments, the chip may include contacts clumped together ingroups and/or disposed in a non-uniform pattern throughout the entirefirst surface of the chip.

The method steps described above are then utilized to provide a finalcompliant chip package, as shown in FIG. 12, whereby the contacts 610are positioned at varying distances from the edge 617 of the chip 600.FIG. 12 shows four different contacts, designated 610A-610D, located inthe peripheral region 612 of the chip 600. The contacts are staggeredwith respect to one another so that contacts 610B and 610D are closer tothe edge 617 of the chip than contacts 610A and 610C. The contacts 610are electrically connected to terminals 675 by bond ribbons 670. Theactual length of bond ribbons 670 may vary based upon the position ofthe contact 610 and the desired position of the terminal 675. Forexample, although contacts 610A and 610C are positioned at a uniformdistance from the edge 617 of the chip 600, bond ribbon 670C is longerthan bond ribbon 670A. As a result, the terminal 675C connected to bondribbon 670C may be positioned at a more central location than terminal675A. The ability to modify the length of the bond ribbons 670 allowsthe terminals 675 to be positioned at an infinite number of differentlocations over the top surface 647 of the compliant layer 640 so thatthe chip package can be reliably interconnected with an external circuitelement, regardless of the location of contact pads on the externalcircuit element.

In a further embodiment, illustrated in FIG. 13, the compliant chippackage includes a supporting element 792 adjacent a semiconductor chip700, with conductive terminals 775 formed over a top surface 794 of thesupporting element 792. The supporting element 792 may include a bar oralternatively a ring having an opening 795 in the center thereof. In thelatter embodiment, the semiconductor chip 700 is provided within theopening 795 so that a first contact bearing surface 720 of the chip 700is substantially parallel with the top surface 794 of the supportingelement 792. The first surface 720 of the semiconductor chip 700preferably includes a passivation layer 730 having openings therein sothat the contacts 710 are accessible through the openings. A compliantlayer 740 having a substantially flat top surface 747 and a bottomsurface and sloping edges 797 therebetween is then formed atop the topsurface 794 of the supporting element 792 and a portion of thepassivation layer 730. The compliant layer 740 preferably fills gaps 755between the peripheral edges of the chip 700 and the support element792. In addition, the compliant layer 740 preferably has ameniscus-shaped top surface so that the transition from the passivationlayer 730 to the compliant layer 740 is smooth. This smooth transitionwill increase the reliability of any bond ribbons formed atop thecompliant layer because the bond ribbons will be gently curved ratherthan kinked. Next, bond ribbons 770 are formed using the techniquesdescribed above, and a dielectric layer 780 is formed over the bondribbons 770 so that only conductive terminals 775 are accessible at thetop of the assembly. In certain embodiments the supporting element 792may include a heat sink and the compliant layer may be formed on the topsurface of flanges extending laterally from central opening in the heatsink.

In still another embodiment, illustrated in FIG. 14, the compliant chippackage includes a flexible dielectric sheet 865, such as a polyimidesheet, secured over the top of the compliant layer 840. The packageincludes a semiconductor chip 800 having a first surface 820 withcontacts 810. A dielectric passivation layer 830, including openings insubstantial alignment with the contacts 810, is then formed over thefirst surface 820 of the chip 800. After the compliant layer 840 hasbeen formed, the flexible dielectric sheet 865 is provided over the topsurface 847 of the compliant layer 840. The flexible dielectric sheet865 generally improves the structural integrity of the package andprotects the compliant layer 840 from external contaminants. Bondribbons 870 are then formed atop the passivation layer 830, thecompliant layer 840 and the flexible dielectric sheet 865. The bondsribbons 870 have first ends which are connected to chip contacts 810 andsecond ends which provide conductive terminals 875 over the flexibledielectric sheet 865. In certain embodiments the dielectric sheet 865 isprovided as a separate sheet which is laminated or secured over the topsurface 847 of the compliant layer 840. In these embodiments, theconductive terminals 875 may be pre-formed on the dielectric sheet 865,with the bond ribbon forming step electrically interconnecting thecontacts 810 and the pre-formed conductive terminals 875. In stillfurther embodiments, the flexible dielectric sheet 865 may be spun ontothe top surface 847 of the compliant layer 840. As such, the edges ofthe spun-on dielectric sheet have radii of curvature which substantiallymatch the radii of curvature of the edges of the compliant layer. Thematched edges provide a smooth transition from the dielectric sheet 865to the compliant layer 840, thereby providing a more uniform surface forforming the bond ribbons 870. A second dielectric protective layer 880may then be formed over the bond ribbons 870 to further protect the bondribbons and electrically isolate the bond ribbons from one another.

In another embodiment, illustrated in FIGS. 15 and 16, a compliant chippackage includes a ground plane 981. As shown in FIG. 15, asemiconductor chip 900 is provided within a central opening 995 of asupporting element 992 so that a first contact bearing surface 920 ofthe chip 900 is substantially parallel with a top surface 994 of thesupporting element 992. A first compliant layer 940 having asubstantially flat top surface 947 and a bottom surface and slopingedges 997 therebetween is then formed atop the top surface 994 of thesupporting element 992. The compliant layer 940 preferably fills gaps955 between the peripheral edges of the chip 900 and the support element992. The sloping edges 997 preferably provide a smooth transitionbetween the top surface 947 of the compliant layer 940 and the chip 900.Bond ribbons are then formed over the compliant layer 940 using thetechniques described above. The sloping edges 997 of the compliant layer940 will increase the reliability of the bond ribbons 970 because thebond ribbons will be gently curved rather than kinked. Next, a secondcompliant layer 941 is formed atop the bond ribbons 970 so as toencapsulate the bond ribbons 970. A dielectric layer 980 is then formedover the second compliant layer 941 and the bond ribbons 970 so thatonly conductive terminals 975 are accessible at the top of the assembly.The ground plane 981 is then provided atop the dielectric layer 980.Referring to FIG. 16, the ground plane preferably includes a highlyconductive material, such as copper, having a plurality of openingstherein. The openings are preferably formed using photolithographic andetching techniques. The openings are sized to fit over the terminals 975and a relatively small portion of the bond ribbon 970 extending awayfrom each terminal 975. The ground plane 981 is assembled to thedielectric layer by aligning the openings 983 therein with the terminals975 and abutting the ground plane 981 against the top of the dielectriclayer 980. The package is then subjected to a curing process so as tocure the compliant layers 940 and 941 and the dielectric layer 980.

FIG. 17 shows yet another embodiment of a compliant chip package whichis similar to that shown in FIG. 15, however, the FIG. 17 embodimentlacks the top dielectric cover layer shown in FIG. 15. Referring to FIG.17, after the second compliant layer 1041 is formed atop the bondribbons 1070, a ground plane 1081, similar to that shown in FIG. 16 isprovided over the second compliant layer 1041. During assembly, theground plane 1081 may be compressed against the second compliant layer1041 so that the ground plane 1081 is slightly sunk into the secondcompliant layer 1041. The ground plane preferably includes openings 1083which are in alignment with terminals 1075 so that the terminals 1075are accessible through the openings 1083. The package is then subjectedto a curing process so as to cure the compliant layers 1040 and 1041.The ground plane 1081 is preferably electrically connected to at leastone of the bond ribbons 1070.

These and other variations and combinations of the features describedabove may be utilized without departing from the present invention asdefined by the claims. For example, the low modulus encapsulant materialshown in FIG. 4 may be used to assemble any of the compliant chippackages shown in FIGS. 7A-14 to provide additional stress relief forthe bond ribbons. In addition, the assembly shown in FIG. 13 may bemodified so as to provide conductive terminals over the central regionof the chip, thereby providing a “fan-in/fan-out” compliant chippackage. Moreover, all of the chip package assemblies disclosed abovemay be assembled on a wafer prior to severing the individual chips fromthe wafer. Thus, the foregoing description of the preferred embodimentsshould be taken by way of illustration rather than by way of limitationof the invention set forth in the claims.

As these and other variations and combinations of the features discussedabove can be utilized without departing from the present invention asdefined by the claims, the foregoing description of the preferredembodiments should be taken by way of illustration rather than by way oflimitation of the invention set forth in the claims.

What is claimed is:
 1. A microelectronic assembly comprising: amicroelectronic element having a first surface including a centralregion and a peripheral region surrounding said central region, saidmicroelectronic element including a plurality of contacts disposed insaid central region; a compliant layer overlying said peripheral regionof said first surface, said compliant layer having a bottom surfacefacing toward said first surface of said microelectronic element, a topsurface facing upwardly away from the first surface of saidmicroelectronic element and one or more edge surfaces extending betweensaid top and bottom surfaces of said compliant layer; and a plurality offlexible bond ribbons disposed over said compliant layer so that saidbond ribbons extend over said top surface and one or more of said edgesurfaces and said bond ribbons electrically connect said contacts toconductive terminals overlying the top surface of said compliant layer.2. The microelectronic assembly as claimed in claim 1, wherein saidconductive terminals overlie the peripheral region of saidmicroelectronic element.
 3. The microelectronic assembly as claimed inclaim 1, further comprising a first dielectric protective layer having afirst surface, a second surface and apertures, wherein the first surfaceof said first dielectric layer is attached to the first surface of saidmicroelectronic element and the apertures are aligned with the contactsof said microelectronic element so that the microelectronic elementcontacts are exposed.
 4. The microelectronic assembly as claimed inclaim 1, wherein said flexible bond ribbons are electrically conductive,each said bond ribbon having a top surface and a bottom surface, whereineach said bond ribbon extends along the edges of said compliant layer.5. The microelectronic assembly as claimed in claim 4, furthercomprising an encapsulant layer attached to the top surfaces of saidbond ribbons.
 6. The microelectronic assembly as claimed in claim 5,wherein said encapsulant layer is selected from the group consisting ofa curable liquid, silicone, flexibilized epoxy, thermoplastic and gel.7. The microelectronic assembly as claimed in claim 1, wherein saidconductive terminals bound the central region of said microelectronicelement.
 8. The microelectronic assembly as claimed in claim 1, whereinsaid compliant layer is selected from a group of materials consisting ofsilicone, flexibilized epoxy, a thermosetting epoxy, fluoropolymer,thermoplastic polymer and polyimide.
 9. The microelectronic assembly asclaimed in claim 1, further comprising a dielectric cover layer over thetop surfaced of said compliant layer and said bond ribbons, wherein thedielectric cover layer has a plurality of apertures therein so that saidterminals are accessible through said apertures.
 10. The microelectronicassembly as claimed in claim 1, further comprising a barrier metal layeratop one or more of the contacts.
 11. The microelectronic assembly asclaimed in claim 1, wherein said microelectronic element includes aplurality of adjacent microelectronic elements arranged in an array toform a corresponding plurality of compliant semiconductor chip packages,said compliant layer including a plurality of regions, each said regionoverlying one said chip.
 12. The microelectronic assembly as claimed inclaim 1, wherein the edge surfaces of said compliant layer are slopingsurfaces that extend in both vertical and horizontal directions.
 13. Themicroelectronic assembly as claimed in claim 12, wherein at least someof said sloping edge surfaces have first transition regions near the topsurface of said compliant layer and second transition regions near thebottom surface of said compliant layer, and wherein both the first andthe second transition regions have respective radii of curvature.
 14. Amicroelectronic assembly comprising: a semiconductor chip having a firstsurface including a plurality of peripheral chip contacts and a centralregion bounded by said peripheral chip contacts, wherein said peripheralchip contacts are positioned at varying distances from an outer edge ofsaid chip; a compliant layer overlying the central region on the firstsurface of said semiconductor chip, wherein said compliant layerincludes a top surface, a bottom surface and sloping peripheral edges; aplurality of electrically conductive bond ribbons, each said bond ribbonhaving a first end that electrically couples to a respective peripheralchip contact, a second send that electrically couples to a respectiveterminal overlying the top surface of said compliant layer, and anelongated portion that extends along the sloping edges between the firstand second ends thereof; and an encapsulant layer overlying said bondribbons.
 15. The microelectronic assembly as claimed in claim 14,further comprising a first dielectric protective layer between said chipand said compliant layer having a first surface, a second surface andapertures, wherein the first surface of the first dielectric layer isattached to the first surface of said semiconductor chip and theapertures are aligned with the chip contacts so that the chip contactsare exposed.
 16. The microelectronic assembly as claimed in claim 15,further comprising a second dielectric layer overlying the top surfaceof said compliant layer and said bond ribbons.
 17. The microelectronicassembly as claimed in claim 14, wherein said terminals are positionedat non-uniform distances from peripheral edges of said semiconductorchip.
 18. The microelectronic assembly as claimed in claim 14, whereinthe peripheral edge of said compliant layer has a first transitionregion near the top surface of said compliant layer and a secondtransition region near the bottom surface of said compliant layer andwherein the first and second transition regions have a radius ofcurvature.
 19. A microelectronic assembly comprising: a semiconductorchip having a first surface including a plurality of peripheral chipcontacts and a central region bounded by said peripheral chip contacts,wherein said peripheral chip contacts are positioned at varyingdistances from an outer edge of said chip; a compliant layer overlyingthe central region on the first surface of said semiconductor chip,wherein said compliant layer includes a top surface, a bottom surfaceand sloping peripheral edges; a plurality of electrically conductivebond ribbons, each said bond ribbon having a first end that electricallycouples to a respective peripheral chip contact, a second send thatelectrically couples to a respective terminal overlying the top surfaceof said compliant layer, and an elongated portion that extends along thesloping edges between the first and second ends thereof, wherein thelengths of at least two adjacent ones of said bond ribbons arenon-uniform.
 20. A compliant microelectronic assembly comprising: asemiconductor chip having a first surface and a plurality of contactsdisposed on the first surface thereof; a supporting element bounding oneor more edges of said semiconductor chip, said supporting element havinga top surface; a compliant layer formed over the top surface of saidsupporting element, said compliant layer including a top surface, abottom surface and edges sloping from the top surface of said compliantlayer to the first surface of said semiconductor chip; conductiveterminals overlying the top surface of said compliant layer; flexiblebond ribbons over the top surface of said compliant layer and extendingon one or more of said sloping edge surfaces of said compliant layer,said bond ribbons electrically connecting said chip contacts and saidconductive terminals.
 21. The compliant microelectronic assembly asclaimed in claim 20, wherein the top surface of said compliant layer issubstantially flat.
 22. The compliant microelectronic assembly asclaimed in claim 20, wherein the first surface of said semiconductorchip and the top surface of said supporting element are substantiallyparallel to one another.
 23. The compliant microelectronic assembly asclaimed in claim 20, wherein a gap is present between said supportingelement and the one or more edges of said semiconductor chip, andwherein a portion of said compliant layer fills said gap.
 24. Thecompliant microelectronic assembly as claimed in claim 20, furthercomprising a dielectric layer overlying said bond ribbons, wherein saidconductive terminals are accessible through openings in said dielectriclayer.
 25. The compliant microelectronic assembly as claimed in claim20, wherein said supporting element is an element selected from thegroup consisting of a bar, a ring and a heat sink.
 26. The compliantmicroelectronic assembly as claimed in claim 20, wherein the one or moresloping edges of said compliant layer have a first transition regionnear the top surface of said compliant layer and a second transitionregion near the first surface of said semiconductor chip, and whereinthe first and second transition regions have radii of curvature forfacilitating formation of gently curved bond ribbons for increasing thereliability of said bond ribbons and said assembly.
 27. The compliantmicroelectronic assembly as claimed in claim 20, further comprising apassivation layer over the first surface of said semiconductor chip.